A signal processing circuit used in, for example, a pressure sensor has an output circuit. A sensor signal of the pressure sensor is output to an external device such as an engine electronic control unit (ECU) through the output circuit. As disclosed in, for example, JP-H9-166620A, the output circuit converts an input signal to an output signal having a voltage within a predetermined voltage range specified by upper and lower clamp (limit) voltages. Thus, the input signal is converted to a suitable voltage signal for the engine ECU.
As shown in FIG. 4, a conventional output circuit 100 includes an amplifier circuit 101, a constant current circuit 102, a filter circuit 103, and resistors 104a-104c. The conventional output circuit 100 is powered by a power supply voltage VCC supplied through the engine ECU. The conventional output circuit 100 converts an input signal VIN applied to an input terminal IN to an output signal VOUT having the voltage within the voltage range specified by the upper and lower clamp voltages and outputs the output signal VOUT to the engine ECU through an output terminal OUT.
The amplifier circuit 101 includes an operational amplifier 101a and resistors 101b, 101c. The amplifier circuit 101 amplifies the input signal VIN with a gain depending on the resistance of the resistors 101b, 101c. 
The constant current circuit 102 includes NPN transistors 102a, 102b, PNP transistors 102c, 102d, and resistors 102e-102i. The transistors 102a, 102b construct a first current mirror circuit and the transistors 102c, 102d construct a second current mirror.
The filter circuit 103 includes resistors 103a, 103b and a capacitor 103c and acts as a resistor-capacitor (RC) filter circuit. The filter circuit 103 eliminates high frequency noise corresponding to electromagnetic compatibility (EMC) noise such as magnetic noise coming from other electronic devices and increases resistance of the conventional output circuit 100 to the EMC noise. The filter circuit 103 has a filter constant depending on the resistance of the resistors 103a, 103b and the capacitance of the capacitor 103c. The filter circuit 103 prevents a noise signal within a certain frequency range to enter the conventional output circuit 100 through the output terminal OUT based on the filter constant.
The resistor 104a is used to determine the clamp voltages, to adjust the amount of current flow to the output terminal OUT, and to detect a break in the conventional output circuit 100. The resistor 104b is used to determine the lower clamp voltage. The resistor 104c is used to determine the clamp voltages and to detect the break in the conventional output circuit 100.
The conventional output circuit 100 operates as follows:
When the power supply voltage VCC is applied to the constant current circuit 102, a base-emitter voltage Vbe of the transistor 102a increases. Then, when the base-emitter voltage Vbe exceeds a transistor turn-on voltage Vf (e.g., 0.7 volts), the transistor 102a is turned on and a collector current I11 flows through the transistor 102a. The amount of the collector current I11 depends on the resistance of the resistor 102e. The resistors 102f, 102g act as a balance resistor for the base-emitter voltage Vbe.
When the transistor 102a is turned on, the transistor 102b is turned on and a collector current I12 flows through the transistor 102b because the transistors 102a, 102b construct the first current mirror circuit. As a result, the transistor 102c is turned on and a collector current I13 flows through the transistor 102c. The collector current I13 is almost equal to the collector current I12. The collector current I12 mirrors the collector current I11 by a first mirror ratio of the first current mirror circuit.
When the transistor 102c is turned on, the transistor 102d is turned and a collector current I14 flows through the transistor 102d because the transistors 102c, 102d construct the second current mirror circuit. The collector current I14 mirrors the collector current I13 by a second mirror ratio of the second current mirror circuit.
The collector current I14 is supplied to the output terminal OUT through the resistor 102i. Thus, the constant current circuit 102 continuously supplies a constant current, i.e., the collector current I14 to the output terminal VOUT during normal operation of the conventional output circuit 100.
The upper and lower clamp voltages are determined as follows:
When a voltage of the input signal VIN is higher than a predetermined reference voltage, an output voltage of the operational amplifier 101a is lower than the voltage of the output signal VOUT. As a result, the collector current I14 and a current flowing through the resistor 101b flows into a junction between the resistors 103b, 104.
In this case, for example, when the resistance of the resistor 104c is much larger than a combined resistance of the resistors 103a, 103b, and 104b, most of the current flowing into the junction flows into the operational amplifier 101a through the resistors 103b, 103a, and 104b. A current I15 flowing through the resistor 104a also flows into the operational amplifier 101a through the resistor 104b. These currents flow into a ground GND through the operational amplifier 101a. Thus, the lower clamp voltage is determined as a voltage drop across the resistors 103a, 103b, and 104b. 
In contrast, when the voltage of the input signal VIN is lower than the reference voltage, the output voltage of the operational amplifier 101a is higher than the voltage of the output signal VOUT. Therefore, the current flows from the output terminal side of the operational amplifier 101a to the output terminal OUT side.
In this case, when a maximum current is output from the operational amplifier 101a, the maximum current, the collector current I14, and the current I15 flows toward the output terminal OUT side, the resistor 101b side, and the resistor 104c side. Thus, the upper clamp voltage is determined as a voltage drop across the resistor 104c. 
The upper and lower clamp voltages specify the voltage range of the output signal VOUT provided to the engine ECU. Therefore, it is preferable that the lower clamp voltage is close to the ground GND (i.e., 0 volt) and the upper clamp voltage is close to the power supply voltage VCC as much as possible.
In the conventional output circuit 100 shown in FIG. 4, whereas the upper clamp voltage may be close to the power supply voltage VCC, the lower clamp voltage may not be close to the ground GND due to the voltage drop across the resistors 103a, 103b, and 104b. Specifically, as shown in FIG. 5, when the voltage of the input signal VIN increases from a voltage Va to a voltage Vb, the voltage of the output signal VOUT changes from 4.8 volts to 0.3 volts.
The lower clamp voltage may be close to the ground GND by eliminating the constant current circuit 102 from the conventional output circuit 100. However, if the conventional output circuit 100 has no constant current circuit 102, the upper clamp voltage may not be close to the power supply voltage VCC.